Digital cable transmission is typically performed with quadrature amplitude modulation (QAM) carriers such as QAM-256 or higher. The carrier frequencies can be arranged in various ways including orthogonal frequency-division multiplexing (OFDM). For instance, a standard can define the data transmission such as, for example, the digital video broadcasting—cable 2 (DVB-C2) standard, which utilizes a large number of OFDM carriers (e.g., thousands) that are each QAM modulated. For example, the standard can provide for an instance with QAM-4096 modulation; that is, a signal constellation of 4096 points. The higher-order the constellation, the more bits per symbol are transmitted and received. However, if the mean energy of the constellation is to remain the same, the points must be closer together and are thus more susceptible to noise and other corruption. This attribute can result in a higher bit error rate for a given carrier to noise ratio, and therefore higher-order QAM can deliver data less reliably than lower-order QAM for constant mean constellation energy.
The large number of OFDM carriers implies that each carrier has a narrow bandwidth and the symbol rate per carrier is low. However, OFDM modulation schemes are generally highly resilient in the presence of reflections within the symbol time of the modulation format. In the case of digital video broadcasting—terrestrial 2 (DVB-T2) that can be as long as 448 microseconds such that the transmission scheme is still effective even with multipath interference over distances exceeding 100 kilometers.
The OFDM scheme used in DVB-C2 originates from the DVB-T2 standard used for terrestrial broadcasting. However, in terrestrial television, broadcasting multipath interference occurs with large differential delays and narrowband interference can occur due to local interference from transmitters. To mitigate these effects an OFDM scheme with a long symbol time and a large number of narrowband carriers is optimal. While this leads to a high peak-average ratio for transmitted power, the transmission medium is linear and therefore is not problematic.
For cable transmission however, reflection distances are limited to a few kilometers at most. The transmission medium is shielded from external interference and the transmission medium has well-behaved characteristics (e.g., compared to terrestrial with multipath interference). Hence, there is no need for the very large number of OFDM carriers used in DVB-T2. On the other hand, the transmission medium contains active elements that cannot efficiently handle a high peak to average power ratio. In DVB-C2, measures are taken with auxiliary carriers that do not carry data to mitigate the peak-to-average power ratio problem of the standard.
Thus, the DVB-C2 standard may not be optimal for cable systems. QAM transmission as defined in DVB-C2 are more robust than required to account for reflections and interference, but sub-optimal with respect to efficient use of the power budget of active transmission equipment. Therefore, an alternate optimized QAM transmission can be implemented that better optimizes the peak-to-average power ratio such that better use can be made of the transmission channel. It should be understood that the methods described can apply to QAM transmissions not isolated to cable transmissions; that is, the presented systems and methods can pertain to any active mediums such as, for example, optical transmitters, amplifiers, digital subscriber lines, among many others.